Retroviruses possess a long region of RNA, termed Leader, between the 5' terminus of their genome and the start codon for gag related polypeptide synthesis. Data from our laboratory and others has suggested that specific nucleotide sequences in the leader are potent negative regulators of down stream coding frame translation. Our laboratory, has also shown that the entire leader plays a role in the efficient initiation of gag synthesis. This study will determine what conditions, if any, promote a functional recognition of negative signals in the leader. Experiments are designed here to adequately determine if the leader RNA is a negative regulator of protein synthesis in vitro and in vivo and whether this negative regulatory capacity has a defined role in the viral life cycle. The efficiency of the Rous sarcoma virus mRNAs will be tested and compared with host cellular mRNAs under a variety of experimental conditions. Experimental assays are designed to measure the functional recognition of the leader in vitro and the subsequent result on downstream gag protein synthesis. Factors will be sought in uninfected and infected chicken cells as well as rabbit reticulocyte lysate which may influence the ability to functionally recognize the leader. Finally, recombinant DNA techniques will be used to place a Retrovirus leader on a marker plasmid gene and ultimately test the effect on translation in vivo, and deletion and co-transfection assays will be used to predict the role of the leader in vivo. It is possible that negative regulation by the Retrovirus leader allows the establishment of an equilibrated viral-cellular protein synthetic regime which is necessary for steady state infection. A second likelihood is that negative regulation could provide a pool of translationally inactive molecules, more likely to be packaged than their actively translated counterpart. In addition to yielding substantial information on the mechanism of initiation of protein synthesis by Retroviruses these experiments should delineate the role of the leader in the Retrovirus life cycle.